Conventionally, a drive device used for the electric power steering device with which the steering operation by a driver is assisted has a one-body structure, in which the motor and the motor controller are combined in one body.
The controller for the drive device disclosed in a patent document, JP 2011-176998 A (Patent document 1) has two substrates that are disposed perpendicularly to the rotation axis of the rotor of the motor (a motor shaft) and a heat sink disposed in between the two substrates. The outer wall of the heat sink has a power module attached thereto at a part of the outer wall that is in parallel with the motor shaft, and the power module molds multiple switching elements to be serving as an inverter circuit. Therefore, the drive device has a heat dissipation capability, dissipating heat of the power module to the heat sink at the power module operation time.
However, the controller making up the drive device of the patent document 1 has two substrates, thereby having an increased dimension along the motor shaft due to the thickness of the electronic components and the substrate supporting those components. Further, the power module is attached on the outer wall of the heat sink that is disposed in parallel with the motor shaft, which creates an excess unoccupied space inside the cup-shape motor cover due to the square shape of the power module.